Synthesis, characterization, and nonlinear optical properties of copper (II) ligand Schiff base complexes derived from 3-Nitrobenzohydrazide and benzyl

A new series of Cu (II) complexes were prepared using Schiff base ligand of N–N′-(1,2-diphenyl ethane-1,2-diylidene)bis(3-Nitrobenzohydrazide). The prepared ligand and Cu (II) complex were characterized using various physicochemical investigations such as X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), and Energy dispersive X-ray analysis (EDX), Fourier Transform Infrared (FT-IR), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}^{13}C$$\end{document}13C Nuclear Magnetic Resonance (NMR), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}^{1}H$$\end{document}1H NMR, Diffuse Reflectance Spectroscopy (DRS), Vibrating Sample Magnetometer (VSM), and Z-Scan technique (Nonlinear optical (NLO) properties). In addition, the prepared samples have been examined for their NLO characteristics with the help of the Density Functional Theory calculations which proved that the Cu (II) Complex is more polarized than Ligand. According to XRD and FESEM results, the nanocrystalline nature of the samples is confirmed. The metal-oxide bond assigned in the functional studies by FTIR. Magnetic studies demonstrate weak ferromagnetic and paramagnetic nature for Cu (II) complex and diamagnetic nature for the ligand, respectively. DRS spectrum exhibited higher reflectance for Cu (II) than the ligand. The band gap energies of the synthesized samples were estimated by employing the Tauc relation and Kubelka–Munk theory on reflectance data and found to be 2.89 eV and 2.67 eV for Cu (II) complex and ligand, respectively. Extinction coefficient and refractive index values were calculated using the Kramers–Kronig method. The z-scan technique was applied to estimate the NLO properties by a 532 nm Nd:YAG laser.

For the first time, the expression "Schiff bases" was used in 1864, since Hugo Schiff, a Nobel Prize laureate, and scientist, prepared the Schiff base (Sb) through a condensation reaction of carbonyl functionality (ketone or aldehyde) and primary amines 1 . The Schiff bases (Sbs) got great attention recently because of their optical application in nonlinear optics (NLO) 2 , fluorescence 3 , electroluminescence 4 , and biological applications like antibacterial activities 5 . With the help of most transition metals, Sbs could easily create a stable complex 6 . As ligands, Sbs successfully used in coordination chemistry due to the broad chelating potential of most metal ions and their facile preparation 7 . Basicity, strength, and steric of the azomethine group affect the Sb complex's stability 8 . Schiff bases are renowned for their diverse catalytic and biological applications, they represent a class of ligands that exhibit a broad spectrum of utility in coordination chemistry 9 . Schiff base derivatives of transition metal complexes have garnered considerable attention as oxidation catalysts for alcohols and alkenes owing to their inexpensive and facile synthesis as well as their remarkable chemical and thermal stability. Schiff bases metal complexes are regarded as a very essential type of organic compounds, which have extensive applications in various biological aspects anti-bacterial, antitumor, antifungal, anti-cancer, anti-tuberculosis, DNA binding, analgesic, antioxidant, and anti-viral properties 10,11 . These tremendous applications of Schiff bases have provided

Results and discussion
Structural investigations. Figure 2a and b exhibit the diffraction peak patterns of the ligand and its complex with Cu (II) obtained from the X-ray diffraction (XRD) technique. The polycrystalline nature of the synthesized samples is confirmed by the existence of different diffraction peaks in the patterns in Fig. 2. In addition, it can conclude that the synthesized samples have monoclinic structures, which are in good accordance with the By applying Debye-Scherrer relation in XRD data for three prominent peaks, the average crystallite size was calculated 39-41 : Here θ is the angle of the diffraction, λ is the wavelength of the X-ray, β (in radians) is the FWHM and indicates the broadening of the diffraction peaks intensity obtained at half of their maximum, D is the mean crystallite size, and K for Cu-K α equals to 0.9 and is a constant.
Moreover, the dislocation density or the number of defects (δ) is expressed as follows 42 : The calculated δ values were 8.80 ×10 14 line/m 2 and 6.25 ×10 14 line/m 2 for ligand and Cu (II) complex, respectively. Compared to ligand, in the complex , the defects and vacancies have reduced regarding the decrement in δ. On the other hand, the smaller δ values for the Cu (II) complex confirm decrement in structural disorder or crystal imperfections, which leads to increasing crystallite size 43 . The obtained crystallographic data   Figure 3 shows that the product is a mixture of 4 crystal phases (green bars). The bars from top to down correspond to the synthesized complex, ligand (S 1 ), ligand (S 2 ), and CuSO 4 , respectively. By comparing the peak intensities of the phases in the product mixture, it shows that the main phase is obtained for the complex (Reference code: 96-222-4482). However, according to the explanations included in the "Supplementary file", the raw materials (ligand and CuSO 4 ) exist in the product mixture and so a composite product has been obtained.     Fig. 4a,b. Particle size and the morphology of FESEM micrographs indicated that in good agreement with the calculated result from the Debye-Scherrer equation. Furthermore, the particle size distribution histogram of the synthesized samples is depicted in Fig. 5a,b.

EDX analysis.
To verify the existence of applied components as well as the distribution of Cu (II) in the Complex structure, the EDX dot-mapping analysis was carried out. The EDX dot-mapping micrographs of the Ligand and Cu(II)-Complex have been depicted in Fig. 6a,b and 7a,b.
1 H and 13 C NMR spectra. Figure 8 demonstrates the 1 H NMR and 13 C NMR spectra of the synthesized ligand.   Fig. 8a, which demonstrates the 13 C NMR spectra of the ligand, the observed peaks in a chemical shift at 191.63 ppm and 147.24 ppm are associated with the carbon of the carbonyl and imine groups (Fig. 9).
On the other hand, in the 1 H NMR spectrum shown in Fig. 8b, the observed broad singlet (bs) peak in the 11.35 ppm area could be associated with NH. In addition, the observed sharp singlet (s) peak in the δ = 8.47 ppm area may be related to H A , H A ′ hydrogens in the 3-nitrophenyl ring. The doublet (d) peak at δ = 8.36 ppm (2H, d, J = 8 Hz, ArH) belongs to the H B , H B ′ protons in the 3-nitrophenyl ring. Also, another doublet peak at δ = 8.12 ppm (2H, d, J = 7.6 Hz, ArH) was attributed to the H C , H C ′ hydrogens in the 3-nitrophenyl ring (Fig. 10).  Table 3.

Functional studies (Fourier transform infrared spectroscopy (FT-IR)). To evaluate the Sb ligand
bonding mode to Cu ion, we studied the ligand and its complex FT-IR spectrum in the range of 400-4000 cm −1 .
The obtained FT-IR spectra for ligand and Cu (II) Complex are exhibited in Fig. 11a-d in various wavenumber ranges. Peaks at 3274 cm −1 and 3205 cm −1 are assigned to the N-H stretching vibrations. These stretching vibrations at vibrational peaks in the synthesized samples determine the hydrogen bond 46 . The absorption band of the amide carbonyl moves toward the lower frequencies due to conjugation with an aromatic ring and appears at 1682 cm −1 and 1686 cm −1 for Cu (II) Complex and ligand, respectively. The absorption band of the imine group is observed at 1663 cm −1 . Peaks at 1528 cm −1 , 1525 cm −1 , and 1348 cm −1 could be linked to symmetry and asymmetry stretching absorption of the nitro groups (Fig. 12). The C-H stretching vibrations are ascertained in the 3150-2900 cm −1 region. On the other hand, in the synthesized Cu (II) Complex sample, the broad and small peak around 3450 cm −1 expressed coordinated and crystalline water 47 . The Cu-O stretching vibration mode was observed in the 590-520 cm −1 region 48 . Table 4, is represented the fundamental FT-IR spectral bands for Cu (II) Complex and ligand.

Magnetic properties.
Recently, investigating the magnetic properties of transition metal polynuclear complexes with Schiff base ligands has attracted enormous interest. These compounds have properties of single-molecule magnetism and single-chain magnetism and are used as a precursor for molecular magnetic materials 57 . In the past decades, Cu (II) complexes achieve great interest because of their important role in the field of molecular magnetic [58][59][60][61][62] . The selection of metal ions with suitable Schiff base ligands for bridging between metal ions in multinuclearity complexes is an important factor in causing magnetic behavior in a complex 63 . A vibrating sample magnetometer (VSM) is utilized to evaluate the synthesized samples magnetically. Figure 13 demonstrates the recorded hysteresis loops (M-H curves) of ligand and Cu (II) Complex under a range of ± 15 kOe magnetic field at room temperature. As can be observed, the synthesized Cu (II) Complex sample indicates a dominant paramagnetic phase that is not magnetically saturated even at 14 kOe applied field, which could be attributed to the paramagnetic nature of the Cu(II) 64 . Also, it has a weak ferromagnetism phase, and its extracted data were summarized in Table 5. On the other hand, for the ligand, we can observe a diamagnetic phase. The saturation   65 . These results represent a good accordance between structural and magnetic properties. Moreover, the appearance of the Cu phase in the synthesized Cu (II) Complex sample (as observed in the XRD patterns) affected the magnetic characteristics and changed them. As mentioned above, variation in crystallite size is a critical point that leads to M s variation 66 . To investigate the magnetic hardness and domain nature of the samples, a critical property is introduced named squareness ratio (K P )expressed as M r /M s ratio. If the K P is higher than 0.5, it expresses a material with a single domain, is highly anisotropic, and is magnetically hard. Visa versa, if the K p is lower than 0.5, the material is randomly oriented and multi-domain 67 . In addition, H c is characterized as a magnetic hardness. Materials falling within the range of 10 3 Am −1 �H c 10 4 Am −1 are classified as soft materials 68 . According to our calculations on Cu (II) complex, the value of H c is approximately equal to 9 × 9 × 10 3 Am −1 , which implies that this sample is a soft room temperature ferromagnetism (RTFM) material. Additionally, the low K P and H c values provide further evidence of the soft ferromagnetic nature of the     Fig. 15, Cu (II) Complex has higher reflectance values compared to the ligand. Moreover, some significant peaks appeared around 350-450 nm because of the transition between conduction and valence bands. The absorption decrement in the UV-visible area could originate from transitions consisting of extrinsic states like existing impurities, defect states, or surface traps 69 . As can be observed in Fig. 16, two electronic transitions are displayed at 355 nm and 432 nm for the ligand, which could be related to the n → π * or π → π * transitions that determine charge transfers at intra-ligand (IL). Besides, the existence of the latter and additional transition at 432 nm represents another order of π-electronic conjugation (π-electrons). On the other hand, these IL transitions in Cu (II) complex can observe at 359 nm, which, in comparison to the ligand, it expressed lower intensities and a redshift 70 . The monoclinic phases and highly crystalline nature of the synthesized samples are verified by the first sharp rise, in the absorbance spectra below 360 nm 71 . Equation (3) initially introduces the Kubelka-Munk theory of reflectance spectroscopy and in the next step, expresses the Tauc relation to obtaining the band gap energies of the synthesized samples as follows 72,73 : Here K and S are the absorptions and scattering coefficients, respectively, h is Planck's constant, n is equal to 1/2 for directly allowed transitions, and υ is the photon frequency.
Through DRS data and the relation mentioned above, we could estimate the band gap energies of the synthesized samples by plotting the (F(R)hυ) 2 verses hυ as indicated in Fig. 17a,b extrapolating each plot to zero value (F(R)hυ) 2 . The band gap energies were calculated as 2.67, and 2.89 for ligand and Cu (II) complex, respectively, which point out inter-band transitions originated from permitted direct transitions. The band gap energies chiefly depend on the degree of structural disorder and structural defects of the materials 69 .
Furthermore, due to the low values of the band gap energies, it seems that the synthesized samples to be exhibited optical conductivity properties. Therefore, for conductivity measurements, the Ligand and Cu (II) Complex samples with a concentration of 19 mM alongside a Dimethylformamide (DMF) solvent were measured Figure 10. Structure of the 3-nitrophenyl rings.  74 . The obtained results were obtained to be 1.5 µSm −1 , 13.7 µSm −1 , and 102 µSm −1 for DMF, Ligand, and Cu (II) Complex, respectively.
Kramers-Kronig studies. The determination of the optical parameters, namely the refractive index (n) and extinction coefficient (k), is a crucial task in the realm of filters, optical devices, and optoelectronic switches 75 . Among the various methods employed for the calculation of optical constants, the Kramer-Kronig (K-K) method, with the aid of MATLAB programming, has proven to be useful for this purpose. The complex refractive index n * (ω) is of paramount importance in the analysis of optical properties and may be expressed in the following relation 76 :  (4) n * (ω) = n(ω) + ik(ω)   The values of n and k for the prepared samples at various wavelengths are depicted in Fig. 18a,b, respectively. The n values for Cu (II) Complex exhibit maximum values in the region of 1.8 eV < E < 2 eV, followed by a sharp increase in the region of 1.5 eV < E < 1.75 eV, which indicates a normal substantial dispersion. It has been observed that n values of the ligand are approximately constant but n values for Cu (II) Complex range widely from 1 to 6 with a decrement in wavelength from 2.25 eV < E < 4 eV. Additionally, a decrement in the n values originates from the preliminary band gap absorption. The lower n values have potential applications in optical devices such as n modifications in desired conductivity and mobility 42 . Moreover, it has been observed that the k-values of the ligand indicate approximately constant same as its n values, but for Cu (II) Complex represents a sharp decrement and reaches zero in, allowing the incident light to pass with negligible loss.
Nonlinear optical properties. The Z-Scan method is used to investigate nonlinear refraction and absorption coefficients in material 77 . In this method, the nonlinear absorption coefficient ( β ) and the nonlinear refractive index ( n 2 ) were evaluated through two closed (CA) and open (OA) aperture structures, respectively. With the www.nature.com/scientificreports/ help of the two photodiodes, the intensity dependant absorption and the fraction of diffracted intensity were measured in the OA and CA, respectively. The Z-scan is carried out through an Nd: YAG laser (532 nm). The laser incident power ( P 0 ) into the synthesized samples was to be 29 mW, and the initial intensity of the laser beam (I 0 )was 36.94 × 10 3 W/m 2 . The values of the β and n 2 could be calculated via the recorded information on photodiodes 2 and 1, respectively. The schematic scheme of the Z-scan setup was represented in Fig. 19 where a Gaussian spatial profile and a continuous wave Nd-YAG laser was applied (wavelength = 532 nm). The beam splitter is named BS in Fig. 19 and the beam propagation is defined to be in the z-axis direction (from negative to positive z). To focus the laser beam, a positive lens at f = 10 cm was used. Photodiodes 2 and 1 measured the  In a nonlinear material, the intensity dependence of the refractive index, n , in a high-intensity laser beam can be expressed by n = n 0 + n 2 I where n 2 and n 0 is called nonlinear and linear refractive index, respectively. This relation points out self-defocusing and self-focusing phenomena in the n 2 < 0 and n 2 > 0 cases, respectively. In the case of n 2 > 0 , the transmittance (T) of the photodiode (1) in Fig. 19 will show a valley and peak when the sample is scanned in the before and after of the focal point of the lens (1), respectively. In the case of n 2 < 0 , the valley and peak position in T in the before and after of the focal point of the lens (1) will be changed. On the other hand, the intensity dependence of the absorption coefficient of the sample in a high intensity could be exhibited by α = α 0 + βI , where, β and α 0 is called nonlinear and linear absorption coefficient, respectively. When the sample is translated on stage, due to this relation, the information of photodiode (2) in Fig. 19 will represent a peak (because of nonlinear saturable absorption (SA)), and a valley (because of nonlinear two-photon absorption) in the β < 0 and β > 0> 0, respectively.
The CA and OA plots of the synthesized samples are exhibited in Figs. 20 and 21. As can be seen from Fig. 20a that expressed the CA Z-scan for the ligand, was initially a peak, and after that, a valley revealed transmittance, along with negative nonlinearity (n 2 < 0) . The corresponding manner observed for Cu (II) Complex in Fig. 21a. In both CA Z-scan Figs. For ligand and Cu (II) Complex (Figs. 20a and 21a), the effect of the lensing is selfdefocusing. On the other hand, Fig. 20b represents the OA Z-scan curve for the ligand where introducing a peak at z = 0 and hence exhibiting SA phenomena. The occurrence of the two-photon absorption is confirmed due to the existence of a valley in Fig. 21b for the Cu (II) Complex.
The n 2 can calculate from the following equation 79 :  where L is the sample length. By fitting the experimental values obtained from CA Z-scan and combining them with the theoretical relation expressed in Eq (10), we can calculate the �ϕ amounts due to the intensity-dependent refractive index. Then n 2 values could be estimated through Eq. (8) 78,79 .
Here z 0 = kω 2 0 /2 called the Rayleigh diffraction length, k = 2π/ ( is the laser beam wavelength) and ω 0 is the laser beam waist on the focal point of the lens (1).
In an OA evaluation, the β values can be calculated through an excellent fitting relation theoretically and experimentally, which is plotted via Eq. (11) and computed via Eq. (12). Obtained results and experimental details of the Z-scan analysis for the Cu (II) Complex and Ligand were summarized in Table 6. As can be concluded, compared to ligand, the Cu (II) Complex exhibited excellent and greater third-order NLO values. This response could be associated with a strong self-focusing effect. Besides, the negative n 2 in an OA may attribute to the high (peak) intensity of transmittance at focus which demonstrated (12) q 0 = βI 0 L eff  Besides, in the absorbance spectra, two electronic transitions at 355 nm and 432 nm were observed for the ligand, which may originate from n → π * or π → π * transitions. With the help of the Kubelka-Munk theory and Tauc relation through reflectance data extracted from DRS, the band gap energies are estimated to be 2.67 eV and 2.89 eV for ligand and Cu (II) complex, respectively. The optical conductivity results were obtained to be 1.5 µSm −1 , 13.7 µSm −1 , and 102 µSm −1 for DMF, Ligand, and Cu (II) Complex, respectively. The calculation of both the refractive index and extinction coefficient, via the Kramers-Kronig studies, indicated maximum values at the same energy for the Cu (II) complex. The Z-scan data reveal that the both ligand and Cu (II) complex exhibit a strong self-defocusing effect. While, the ligand showed a negative and saturable, and Cu (II) complex exhibited a positive and two-photon nonlinear absorption, respectively. The magnitude of the third-order nonlinear susceptibility of the complex was examined to be 4.68 × 10 -3 esu and 1.61 × 10 -3 esu for Cu (II) complex and ligand, respectively. The obtained results of the Z-scan analysis revealed that the Cu (II) complex had improved the NLO characteristics of the ligand. A good synchronicity is observed between DFT-based calculations and experimental results. These results verified the potential usage of these materials (especially Cu (II) complex) in nonlinear photonic, nonlinear optical, and optoelectronic applications such as wavelength conversion and optical switching.

Data availability
All data generated or analyzed during this study are included in this published article.